These days, improvements in the fuel efficiency of automobiles are required from the viewpoint of global environment protection. On the other hand, the maintenance and improvement of collision safety of vehicles are required. In order to satisfy these requirements, high-strength, lightweight car body structures are being developed. For example, the use of a thin-walled, high-strength steel sheet is promoted mainly for a framework member such as a frame or a pillar.
A pair of rear frames, which are each a framework member and are provided at left and right in a rear portion of a vehicle, absorb collision energy by bringing about bending deformation due to an impact load from the rear side of the vehicle. Technology to improve the impact energy absorption capacity by such rear frames is being developed.
For example, Patent Literature 1 below discloses a technology regarding a rear frame in which a bead section is provided in a bottom wall section and a flection section that flexes to the inside is provided in a vertical wall section. By this technology, the bending deformation load generated by an impact load can be increased. That is, the capacity of collision energy absorption by the rear frame is improved. Further, Patent Literature 2 below discloses a rear floor structure that includes a rear floor panel formed of a soft steel sheet and rear frames formed of a high-tensile steel material having a higher strength than the rear floor panel. By this technology, the amount of collision energy absorbed can be increased while the amount of deformation of the car body with respect to the impact load is kept small.